Nichrome, manganin and constantan are widely used in wire bound standard resistors because:

CONCEPT:

Electrical Resistance:

• The measurement of the opposition of the flow of electric current through a cross-section of the material is called the electrical resistance of that material.
• It is denoted by R.
• The relationship between resistance and temperature is given by,

⇒ R = R_o(1 + αΔt)

Where R = resistance at a reference temperature, α = temperature-coefficient of a conductor, and Δ T = changes in temperature

• For metals, α is positive and for semiconductors, α is negative.
• Some materials like Nichrome (which is an alloy of nickel, iron, and chromium) exhibit a very weak dependence of resistivity with temperature.
• Manganin and constantan have similar properties. These materials are thus widely used in wire-bound standard resistors since their resistance values would change very little with temperatures.

EXPLANATION:

• Some materials like Nichrome (which is an alloy of nickel, iron, and chromium) exhibit a very weak dependence of resistivity with temperature.
• Manganin and constantan have similar properties. These materials are thus widely used in wire-bound standard resistors since their resistance values would change very little with temperatures.
• Hence, option 2 is correct.